1. Field of the Invention
The invention relates generally to metal halide lamps and specifically to the construction of such lamps with alumina ceramic reflectors.
2. Description of the Prior Art
Werner Block, et al., describe a metal halide high-pressure discharge lamp in U.S. Pat. No. 4,686,419, issued Aug. 11, 1987. A glass bulb in the general shape of a cylindrical tube with electrodes protruding at the two ends is filled with a noble gas, like argon, and a small amount of mercury. A halogen component of the metal halide comprises iodine and/or bromine. An average light density in excess of thirty kilostilb with a specific arc power of 400 to 5,000 watts per centimeter is obtained over an average lifetime of 250 hours is reported. Such lamps find application in optical projection systems when used in combination with a reflector. FIG. 3 of Block, et al., is repeated herein as FIG. 1.
FIG. 1 illustrates a cross-sectional view of a lamp 10 with a reflector 13 "securely connected and assembled" with a bulb 14. The bulb 14 has a power rating of 270 watts, and is coaxial with the axis of the reflector 13. An electrode shaft 15 is secured by a "suitable cement 16" to a ceramic base 17. Another electrode shaft 18 is attached to the edge of the reflector 13 by a pair of copper strips 19 and 19a, fitted into a ceramic terminal ring 20. An electrical connection to the copper strips 19 and 19a provide one terminal for a power supply. The copper strips 19 and 19a further serve as cooling fins. A pair of electrodes 21 and 22 are connected to a pair of molybdenum foils 23 and 24 that are melt-sealed into the lamp shafts 15 and 18. The foils 23 and 24 are connected to a pair of electrical terminals 25 and 26. An electrical connection 27 is connected to the foil 23 and to the base 17. Another connection 28 is fitted into the ring 20 and electrically connected to the copper strips 19 and 19a. Thus, the terminals 27 and 28 provide for connection to a power supply. The reflector 13 is described as being a "mirrored ceramic or glass".
Commercial products that generally fit the description of FIG. 1 have wire leads with high-voltage insulation and an industry-standard connector. Typical uses include small spot lights used at rock concerts. Such conventional lamps typically have short lives and suffer from cracking of their reflectors. Many different commercial producers have tried a variety of materials and configurations in the construction of their respective reflectors, albeit none with much success. Osram (Germany) markets a "HTI" lamp which has a glass (borosilicate)reflector with a dielectric reflective coating that allows infrared radiation from the bulb to pass through the glass. Such reflectors have been observed to prematurely crack under the heat generated by its metal halide bulb. Ushio (Japan) markets a lamp with glass ceramic reflector and a dielectric reflective coating that is very robust, but costs more than a ceramic reflector. Reflectors that pass-through the infrared radiation and that do not absorb it are responsible for the uncontrolled heating of boxes and fixtures that house such lamps. Such housings exacerbate such heating by being typically painted black.